The present invention relates to processes for the preparation of mixed persalts capable of liberating hydrogen peroxide in solution, and more particularly, this invention relates to processes for the preparation of mixed persalts which are stable in the dry state in alkaline washing mixtures, as well as to the use of such persalts as oxidizing agents in alkaline detergent mixtures.
The depletion of boron minerals and the increasingly stringent standards for the prevention of pollution is compelling the manufacturers of detergent products to seek materials which can replace perborates in detergent and washing powders. Sodium percarbonate, 2 Na.sub.2 CO.sub.3 .multidot.3 H.sub.2 O.sub.2, seems destined to be such a material, and it also possesses a further advantage over the perborate in that it dissolves more rapidly in water.
However, sodium percarbonate exhibits a drawback because it is much less stable than the perborate. As well as during its use for washing, in cartons of detergents sold at retail, and during detergent manufacture in spray towers or during the time of delivery to the manufacturer of the detergent, the sodium percarbonate undergoes decomposition, with a loss of active oxygen, very distinctly greater than the decomposition of perborate under the same conditions. The elimination of impurities, such as heavy metals, which catalyze the decomposition reaction, permits curing of the problem caused by instability of the percarbonate in all of these circumstances, with the exception of its stability in alkaline washing mixtures. While the perborate loses less than 4% of its active oxygen during four months of storage at ambient or room temperature in detergent compositions, a percarbonate which is stable under other conditions, loses at least 20% of its active oxygen.
Numerous solutions have been proposed for alleviating this stability problem of percarbonate in alkaline mixtures. Interox French Pat. applications Nos. 73/27523, 73/35424, 73/3115, and 74/2642 describe the stabilization of percarbonate by covering it with organic polymers to isolate the percarbonate particles from the other constituents of the detergent. This coating according to the Interox patents is carried out on dry percarbonate particles and involves spraying the surface of the particles with an enrobing agent, followed by drying.
DuPont French Pat. No. 2,227,320 suggests another answer to the problem, which method involves spraying a solution containing Mg++ ions on the powdered detergent mixture and then going on to dry it before introducing the percarbonate. The impurities which catalyze the composition of the percarbonate are themselves coated by the magnesium product by reaction between the Mg++ ions and the alkaline constituents of the detergent.
All of these answers to the problem are extremely costly and raise the price of the thus-stabilized detergent products to a prohibitive level. There accordingly exists a considerable commercial need to provide economical processes for the preparation of oxidizing agents which can replace perborate in detergent compositions and at the same time possess dry stability which is at least as good as that of the perborates.